1. Field of the Invention
The present invention relates to data transmission technology. More particularly, the present invention relates to a pre-coding/decoding method and apparatus for data transmission between a transmitting end and a receiving end.
2. Description of Prior Art
Radio services are now of increasing importance, and accompanied by a growing demand for higher network capacities and performances. The conventional technical solutions, such as bandwidth augmentation, modulation optimization, and even code reuse, have limited potential for improving spectrum utilization efficiency. MIMO (Multiple Input Multiple Output) systems adopt antenna arrays and space reuse technology to improve bandwidth utilization efficiency. In many practical applications, channel parameters are obtained via a feedback channel from the receiving end to the transmitting end (assuming the feedback delay is far less than the channel coherence time).
In the TDD (Time Division Duplex) system, the estimated values for a channel in the receiving mode can be used in the transmitting mode if the space between data receiving and transmitting is completed in the ping-pong time, (assuming the ping-pong time is far less than the channel coherence time). This leads to a question: how to use these channel estimates to optimize transmission solutions for transmitters and to design optimal receivers accordingly. Currently, the research is mainly on linear and nonlinear optimized pre-coding techniques.
The nonlinear pre-coding scheme offers a better performance, but is far more complicated than the linear scheme. Linear pre-coding technology is thus becoming the focus of research. Linear pre-coding technology makes full use of part or all of Channel State Information (CSI) to obtain as much beam forming gain as possible. Singular Value Decomposition (SVD) is the most commonly used method in the linear pre-coding technology. In theory, SVD-based linear pre-coding technology can achieve a transmission rate reaching the limit of channel capacity. SVD-based linear pre-coding technology requires that the transmitting end knows as much CSI as possible, and the basic principle is discussed below.
Consider an MIMO system with Nt transmitting antennas and Nr receiving antennas in a flat fading channel. Let x be a complex vector of data symbols, H be a Nr×Nt channel matrix which complies with Rayleigh distribution, and n be Additive White Gaussian Noise (AWGN). The vector y of the received symbols at the receiving end is:y=Hx+n  (1)
where H represents a channel matrix, x represents a transmission signal, and n represents AWGN.
SVD is used to decompose the channel matrix H into 3 matrixes, SVD{H}={U,Σ,V}, where U and V are unitary matrixes, and Σ represents a singular value diagonal matrix of the channel matrix H arranged in a descending order. The SVD expression of the channel matrix H is:H=UΣV*  (2)
where U represents a unitary matrix including a left eigenvector of a channel matrix H, V represents a unitary matrix including a right eigenvector of the channel matrix H, and ‘*’ represents a conjugate transpose operator.
FIG. 1 illustrates a block diagram of the SVD-based MIMO according to the related art.
Referring to FIG. 1, the data symbol vector x 100, after being multiplied by the pre-coding matrix V 102 at the transmitting end, is sent out through Nt antennas 104. The data signals arrive at the receiving end via MIMO channels. The receiving end uses Nr antennas 106 to receive the signals and uses the pre-decoding matrix U* to remove any influence from the channels. The received vector y 110 can be expressed as:
                                                        y              =                                                U                  *                  H                  ⁢                                                                          ⁢                  V                  ⁢                                                                          ⁢                  x                                +                                  U                  *                  n                                                                                                        =                                                U                  *                  U                  ⁢                                                                          ⁢                  Σ                  ⁢                                                                          ⁢                  V                  *                  V                  ⁢                                                                          ⁢                  x                                +                                  U                  *                  n                                                                                                        =                                                Σ                  ⁢                                                                          ⁢                  x                                +                                  U                  *                  n                                                                                        (        3        )            
where U represents a unitary matrix including a left eigenvector of a channel matrix H, V represents a unitary matrix including a right eigenvector of the channel matrix H, x represents a vector of a transmitting signal, n represents AWGN, and ‘*’ represents a conjugate transpose operator.
If the transmitting end has an already known CSI, the transmitting end can use matrix algorithm to pre-code the signals to be transmitted. This can simplify receiver performance. In the TDD system, uplink and the downlink share the same frequency band. According to the reciprocal principle of uplink and downlink channels, the transmitting end can therefore use a preamble in the uplink or Uplink Sounding (UL Sounding) to estimate the CSI in the downlink. Unfortunately, because delays due to necessary processing have to be considered, the channel estimate of the uplink cannot be directly applied to the downlink. Moreover, the number of slots in the uplink may be less than those in the downlink. This is due to asymmetrical services resulting from such multimedia services as data streaming services, data download services, FTP, P2P, online video and digital broadcasting, as shown in FIG. 3. Therefore, deterioration in system performance may be caused if the transmitting end or the base station uses outdated uplink channel estimates in pre-coding.
In the TDD mode of the existing Institute of Electrical and Electronics Engineers (IEEE) 802.16e Standard or the IEEE 802.16m Standard under research, there is still a problem of uplink and downlink service asymmetry between the base station and the mobile user due to multimedia services.
FIG. 2 shows a frame structure 200 of several switchovers of symmetrical services according to the related art, where downlink services 202 are denoted by the sign “↓”, and uplink services 204 are denoted by the sign “↑”.
Referring to FIG. 2, the whole burst service is divided into several frames, each frame containing several slots. Each frame is divided between downlink data services and uplink data services. Assuming that in one frame the channel experiences a slow change procedure, the transmitting end can follow the reciprocal principle of between uplink and downlink channels and use a pilot, a preamble or UL Sounding in the uplink to estimate the CSI of the downlink.
FIG. 3 shows a frame structure 300 of several switchovers of asymmetrical services according to the related art.
Referring to FIG. 3, there will be more downlink services 302 than uplink services 304 when a mobile terminal requires multimedia services. This results in asymmetry between the uplink and the downlink, with delays between them. The downlink CSI will differ from the CSI estimated using uplink signals such as pilot or UL Sounding signal. If the transmitting end or the base station uses outdated uplink CSI in pre-coding, it will cause inaccuracy in pre-coding and reduce system performance.
A non-patent document by Antti Tölli, Marian Codreanu, and Markku Juntti (Compensation of Non-Reciprocal Interference in Adaptive MIMO-OFDM Cellular Systems, IEEE Transactions on Wireless Communications, Vol. 6, No. 2, pp. 545-555, February 2007) proposes a method which mainly uses the feedback from the receiving end to compensate for the difference between the estimated channel value of the uplink and the actual channel value of the downlink. However, the shortcoming of the above method is that, in the TDD system, system resources for uplink should be occupied to compensate for the difference between the estimated channel value of the uplink and the actual channel value of the downlink. Given the shortage of channel resource in uplink and the increased complexity at the receiving end, it is not feasible to use this method in practical applications.
Therefore, there is a need for a method in which the difference between the estimated channel value of the uplink and the actual channel value of the downlink can be compensated by using downlink channel resource (such as DL Sounding signal) in the TDD mode, especially in asymmetrical service channels. Such a method can downlink channel resource to optimize channel estimation and SVD pre-coding at the transmitting end. This can reduce occupation of uplink channel resource, and thus not cause increased complexity at the receiving end. Such a method can also improve the pre-coding performance of the transmitting end and the pre-decoding accuracy of the receiving end, and can expand the bandwidth utilization efficiency of the system as much as achieving the channel capacity.